U.S. Pat. No. 3,692,551 (incorporated herewith by reference), issued Sept. 19, 1972 to W. R. Weaver, discloses a pressure molding process utilizing a non-detrudable molding core which is made by casting a molten salt and sand mixture. As disclosed in the patent, the use of salt mixtures having a relatively low melting point produces a core having extremely smooth porcelain-like surface characteristics which are highly desirable in pressure molding processes. In addition, the use of sand as part of the core composition results in the core having high compressive strength, a requirement for die casting and for molding articles out of plastics.
According to the patent, the cores are made by dip-coating or surface-coating solid sand cores in a molten salt bath or, alternatively, by mixing sand into a molten salt bath and casting the resulting liquid mixture into core form.
The patent discloses the removal of the core from the finished article by dissolving, flushing, and leaching out the core as a solute in water or in weak acid solution or by mechanical vibration.